To cool to even lower temperatures, adiabatic nuclear demagnetization is the most widely used technique, and it is capable of reaching microkelvin temperatures in bulk metals [1]}, [2]}. In this single-shot method, first the nuclear spins in an appropriate paramagnetic metal – the nuclear refrigerator (NR) – are polarized by applying a large magnetic field and then removing the heat of magnetization, typically by using a dilution refrigerator over a number of days. Subsequently, the field is ramped down adiabatically to a small but finite final field. Ideally, this reduces the temperature of the nuclear-spins subsystem by the same factor as the reduction in field. The heat leaking in from the environment is absorbed by the NR and slowly destroys the polarization of the nuclear spins which gives them a finite cooling power. This technique has been used very successfully and broadly, for example in producing record-low nuclear-spin temperatures down to 280 pK [3]}. Because electrons in the NR couple directly and strongly to the nuclear spins, demagnetization can be very effective in lowering the temperature of electrons in nanoelectronic devices with on-chip NRs that cool the device directly.
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